Bell shaped annealing furnaces are typically used for batch coil annealing of metal strip wound into coils. A plurality of coils are vertically stacked one on top of the other (separated by diffuser plates) onto a fixed annealing stand or base. The coils are covered with an inner cover which is sealed (removably) to the annealing stand. An outer cover, typically sealed also to the annealing stand, surrounds the inner cover. The outer cover typically carries burners or a source of heat which heats the outside surface of the inner cover which in turn radiates the heat to the coils therein. The coils are heated until they reach their transformation temperature and they are held at this transformation temperature until temperature uniformity has been achieved throughout the coil. At that time the coils are then cooled to achieve the desired annealing. As noted, the inner cover is sealed to the base and a protective atmosphere is usually maintained within the inner cover while heating to a transformation temperature. This inner atmosphere is typically a HNX (hydrogen-nitrogen) composition. The invention can also be used with pure hydrogen.
Cooling of the coils after heating is the longest stage of the annealing cycle. Typically, it takes twice the time to cool the work when compared to the time it takes to heat the work. The availability of annealing stands or furnace bases is often limited in the mill and plant capacity is thus set by the cycle times and available number of bases. Cooling time reduction is thus the easiest way to increase base throughput and thus plant capacity.
In the batch coil annealing process as initially practiced, after the work was heated, the outer cover was simply removed and heat was exchanged between ambient air and the inner cover to accomplish cooling. Today cooling is performed usually with a cooling cover. Most cooling covers are updraft type covers. These covers pull air over the face of the hot inner cover and exhaust the heated air at the top. Methods have been tried to improve air flow turbulence thus increasing cooling of the inner cover but often, especially with sand seals, problems are encountered.
Other cooling methods use water. Water is either directly sprayed on the cover at the top so it flows down the inner cover. Alternately, water can be sent through heat exchange coils in the base where it cools the protective atmosphere. One type of a heat exchange coil used for cooling mounted in the base is marketed under the brand name "INTRAKOOL". Use of water requires extra measures and additional operating equipment and further adds to maintenance needs. The problems typically encountered with internal exchangers is simply failure of the heat exchanger resulting from thermal shock which occurs when water is pumped into a coiled hot tube. Cooling the inner cover by pouring water over the inner cover also presents operational and maintenance problems. Thus the trend is to use air cooling covers in which air is directed against the inner cover and high heat transfer rates are achieved.
One type of such cover was developed by the assignee of the present invention and this invention is viewed as an improvement over the assignee's earlier design. In that design, a structure and concepts similar to that disclosed in Hemsath U.S. Pat. No. 4,891,008 was employed. More specifically, a plenum chamber was formed in the top end of the outer cover and extending from the top end of the outer cover was a plurality of circumferentially spaced, longitudinally extending distribution tubes. Each tube, was in fluid communication with the plenum chamber at one axial end and each tube had a plurality of orifices drilled at longitudinally spaced increments along its length. When ambient air was pulled by a fan into the plenum chamber and pumped into the distribution tubes, the orifices metered the ambient air out of the distribution tubes as free standing jet streams of high velocity which impinged the cooling covers to further enhance or speed up the cooling of the inner cover. The jet pumps developed from the distribution tubes were effective to significantly decrease the cooling time. However, some initial operational problems were encountered which led to the development of the invention set forth herein. Of some concern was the cooling pattern developed on the inner cover by the variation of jet velocity resulting, to some extent, from the fundamental orifice-longitudinal tube arrangement. Even so, the cooling rates achieved by the jet streams in the early design was far superior to that achieved by other cooling covers in use. However, another problem, which has also plagued other cover installations, resulted from the fact that the large flow of cooling air impinging the inner cover exerts a positive pressure at the base of the cooling cover. Should the base of the cover be an elastomer seal positively clamped by a mechanical hydraulic device compressing the seal, the positive pressure exerted at the base of cooling cover does not present any significant concern. However, if the seal for the cover is the traditional sand seal, then the positive pressure within the cover will disturb the sand and break the sand seal. This presents a major dust problem which the present invention overcomes.